FR2772888A1 - IMPROVEMENT TO SOLID FUEL BURNERS - Google Patents

Abstract

The technical sector of the invention is that of the manufacture of furnace burners. The present invention relates to solid fuel burners, and in particular to pulverized coal burners capable of equipping ovens, in particular rotary kilns. solid fuel burner according to the invention, comprises at least one annular section for supplying solid fuel and at least one annular duct (9, 11) for supplying air, which is arranged substantially coaxially and outside the duct d 'solid fuel supply; said burner comprises at least two obstacles (17) provided in the fuel supply duct, which divide the duct into at least two channels (71, 72, 73, 74, 75), in the vicinity of the outlet of the drove.

Description

Improvement to solid fuel burners The present invention is

  relating to solid fuel burners, and in particular to pulverized coal burners capable of equipping ovens, in particular rotary ovens.

  The technical sector of the invention is that of the manufacture of oven burners.

  The present invention is particularly applicable to a burner for rotary tubular ovens, which is equipped with: - a first central conduit for supplying liquid fuel, provided with a flame stabilizer which has openings for the passage of a part of the primary combustion air, circulating in a second (annular) supply duct, one end of which is arranged coaxially and externally to the first duct, - a third (annular) duct supplying pulverized solid fuel, one end of which is arranged coaxially and externally to the second duct, - a fourth (and possibly a fifth) (annular) duct for supplying primary air, one end of which is

  arranged coaxially and externally to the third conduit.

  The object of the present invention is to provide a burner with

  solid fuel that is improved.

  The object of the present invention is in particular to reduce the

  production of nitrogen oxides resulting from combustion.

  The solution to the problem posed consists in proposing a solid fuel burner which comprises at least one annular section duct for supplying solid fuel and at least one annular air supply duct, which is arranged substantially coaxially and outside the duct d 'solid fuel supply; said burner comprises at least two obstacles provided in the fuel supply duct, which divide the duct into at least two channels, in the vicinity of the outlet outlet of the duct, that is to say of the burner nozzle , so as to cause the incident stream of solid fuel to separate into at least two streams to allow the formation of at least two flames. It has been found that the formation of several combustion flames resulting from the formation of several solid fuel streams at the outlet of the burner nozzle, results in a reduction in the formation of nitrogen oxide, this reduction being so surprising in particular increased when the invention is applied to a burner equipped with several axial air and air supply ducts

  swirling which are arranged outside the solid fuel supply duct.

  According to preferred embodiments of the invention - said obstacles are in the form of ribs or thick partitions, of variable cross section, preferably of profile and continuously variable cross section, for example wedge-shaped and elongated along of the common longitudinal axis of the ends of the pipes, to limit or avoid abrasion of the pipe and / or obstacles by solid fuel particles, to avoid a loss of speed and the need for overpressure of the solid fuel , while separating the flow of solid fuel into several clearly separated streams at the outlet of the nozzle; the height (measured radially) of the channels separated by said obstacles from the solid fuel supply duct, is greater than the height of the portion of the duct situated upstream of the obstacles, in order to limit the increase in speed resulting from the decrease the section for the free passage of solid fuel in the channels, for example to maintain the speed of the substantially identical solid fuel in the channels and in the upstream part of the supply duct; - Said burner comprises at least three and preferably at most twelve obstacles, in particular three to six obstacles substantially regularly distributed along the periphery or circumference of the annular duct and arranged near the outlet orifice of the supply duct to allow the formation of at least three flames of similar or identical shape and size and distributed around the common longitudinal axis with rotational symmetry - the portion of the solid fuel supply duct comprising said obstacles and said delimited channels by obstacles, is of substantially flared conical shape, and is in particular provided with a substantially cylindrical internal face and with a substantially frustoconical or divergent external face in the direction of the fuel outlet orifice; o10 - said burner comprises a said axial air supply duct, ejected through an outlet orifice in a direction substantially p parallel to the common longitudinal axis of the ducts, and guided by partitions or ribs or fins provided in the duct, which is arranged around the solid fuel supply duct, which axial air supply duct has partitions or ribs thick, the number of which is at least equal to the number of obstacles provided in the solid fuel supply duct, to form at least as many axial air passage channels as the burner has solid fuel passage channels , in order to allow the formation of at least one separate air jet, associated with each stream of the outgoing stream of solid fuel; - the number of channels and partitions provided in the axial air supply duct, is a multiple of the number of channels and obstacles provided in the solid fuel supply duct, to allow the formation of several (at least two ) axial air jets, associated with each of the jets or streams of solid fuel leaving the burner nozzle; the channels delimited in the air supply duct by the partitions and / or the air outlet orifices and / or the air jets leaving these channels are angularly offset with respect to the channels delimited by the obstacles of the solid fuel supply duct and / or relative to the solid fuel outlet orifices and / or to the solid fuel streams leaving the nozzle, in the direction of rotation imparted to the fuel and to the flame by an air flow swirling delivered by an air supply duct provided with fins or deflectors; - The channels and / or the axial air jets leaving these channels are inclined, for example divergent with respect to the common longitudinal axis of the burner nozzle and the supply conduits; - Said burner further comprises an annular external peripheral pipe for injecting an oxygen-poor gas, outside the outlet orifices of the fuel and primary air channels. According to another aspect, the invention consists implementing a process for operating a furnace equipped with a burner, in which the average speed of solid fuel in the duct and in the channels separated by obstacles is maintained at a value at least equal to 20 meters per second and at most equal to 30 meters per second, and in which one delivers around the solid fuel stream a part of the primary air in swirling form and a part of the primary air in substantially axial form and maintaining a flow d axial air greater than the swirling air flow, for example an axial air flow

  close to twice the swirling air flow.

  The advantages of the invention will be better understood from

  through the following description which refers to the accompanying drawings, which

  illustrate without any limiting character preferential modes of

realization of the invention.

  In the drawings, identical or similar elements bear,

  unless otherwise indicated, the same references from one figure to another.

  Figure 1 illustrates schematically and in perspective view an embodiment of a solid fuel supply duct forming part of the burner nozzle, equipped at its end with five obstacles in the form of triangular profiled corners separating the vein solid fuel in five veins flowing in canals

delimited by these obstacles.

  Figure 2 is a partial longitudinal sectional view of the nozzle

  a burner according to the invention.

  Figure 3 is a front view and is a view along III of the figure

  2, the nozzle of a burner according to the invention.

  The burner, illustrated particularly in FIGS. 2 and 3, is equipped with a first central conduit 3 for supplying liquid fuel, provided with a flange stabilizer 16 of flange which has openings for the passage of a portion. primary combustion air, circulating in a second annular supply duct 5, one end of which is arranged coaxially and externally to the first duct 3,10 - of a third annular duct 7 supplying pulverized solid fuel, of which one end is arranged coaxially and externally to the second duct 5, - a fourth and a fifth duct 9, 11 annular of primary air supply, one end of which is disposed

  coaxially and externally to the third conduit 7.

  The conduits 3, 5, 7, 9, 11 are arranged along a longitudinal axis 1 common to these different conduits, near their end (in the left part of FIG. 2) constituting the burner nozzle, allowing the production of a flame; the products (air and fuel) flow in these conduits according to arrow 15, that is to say from right to left with reference to FIG. 2; these conduits 3, 5, 7,

  9, 11 are delimited and separated from each other by tubular walls 4, 6, 8, 10 respectively.

  In the burner illustrated in FIGS. 2 and 3, which is as described in patent EP 421 903, the fourth and fifth air supply ducts 9, 11 serve to deliver a first flow of axial combustion air, this is that is to say moving substantially parallel to the common longitudinal axis of the ends (forming a nozzle) of the various conduits, and used to deliver a second flow of combustion air set in rotation, that is to say moving substantially in vortices along a helix whose axis coincides with said axis

longitudinal common 1.

  As described in patent EP 421 903, a central, so-called dead zone 200 is introduced either directly around the central fuel supply duct 3 and / or radially inside the annular fuel supply duct 7 solid, an amount of primary air of not more than 20% of the total primary air, so that in the central area no combustion occurs

  notable fuel; thanks to the presence of a central dead zone, combustion begins at a greater distance in front of the center of the nozzle 2 of the burner in comparison with the previous methods and burners10.

  Thus, seen in longitudinal section, starting from the center of the front end of the nozzle, the envelope of the resulting flames

  has a central zone which extends radially forward, in which there is practically no significant combustion of the fuel with the oxygen of the air; a primary air flow in this central area, which is below 20% and preferably below

  below 10% of the total primary air, avoids a return of the combustion products, without providing a lot of oxygen which would decrease the central zone rich in fuel.20 Thanks to the presence of a central dead zone enlarged, the burner adjustment range is increased to less than 10% of the rate

  maximum, for which the burner is designed; since such a burner must operate with solid and pulverulent fuels, after a brief preheating with oil, it can already operate with solid fuel.

  The proportion of primary air which must be used to stabilize the flame can be reduced by 2 to 10% and, preferably, below 6% compared to the overall quantity of combustion air; the reduced consumption of primary air saves energy for equal performance, and allows the reduction of the proportion

  nitrogen oxides in the flue gases.

  The radii of the central opening of the flame stabilizer 16 and of the annular orifice for the innermost primary air stream are chosen, so that the interior primary air stream is at a distance from axis 1 which corresponds to at least twice the central opening of the flame stabilizer. The central opening of the flame stabilizer corresponds substantially to the orifice5 of the nozzle of the central fuel conduit 3; this prevents the liquid fuel from coming into contact too early with the oxygen in the primary air streams leaving the ducts 9, 11. The annular ducts 9, 11 for the primary air supply have conical walls and these walls as well as the concentric tubes connected to these can be moved axially relative to each other; it is thus possible to adjust the free cross section of the annular passage; the ends of each duct 9, 11 are cylindrical in order to avoid divergent flow at the outlet of the nozzle; in the annular axial air supply duct 11, radial partitions 19 are arranged intended for the axial orientation and for the circulation of the primary air in

separate channels.

  These partitions contribute to an additional axial orientation of the corresponding primary air and also increase the axial outlet speed, by the fact that they reduce the free section of the annular duct and that they divide it into a plurality of channels 111 .. .114 arranged around a ring. Some of these channels may be closed or be adjustable. To this end, the partitions can be made wide enough in the peripheral direction so that they close off a channel at least in part or that they correspond to a closed channel; increasing the axial air flow speed

helps stabilize the flame.

  The flame stabilizer 16 is offset rearward (with reference to the direction of circulation) relative to the outlet orifices, 90, 110 of the main part of the primary air and of the solid fuel, by fixing the flame stabilizer to the envelope tube of the

  burner rod, which is axially movable.

  With reference to FIG. 1 in particular, the duct 7 for supplying solid fuel comprises an upstream part situated on the right in this figure, which is provided with an annular cross section; due to the presence of the five obstacles 17 in the form of a triangular profiled wedge, the conduit 7 is divided into five channels 71, 72, 73, 74, which are each delimited by, on the one hand, the external face 6A of the wall 6 separating the conduit 7 from the conduit 5, on the other hand the internal face 8A of the wall 8 separating the conduit 7 from the conduit 9, and also the lateral faces 173 of the obstacles 17 in the form of a wedge, which extend from their rear end 170 tapered or profiled, and whose cross section widens or increases from their leading edge 170 to o10 their terminal section 171 corresponding to the end or front face of the nozzle; in order for example to maintain a substantially constant solid fuel passage section between the upstream part of the completely annular section conduit and the downstream part provided with obstacles and divided into five channels, the internal radius of the stream remaining for example constant due to a cylindrical surface 6A, the external radius 21 corresponding to the radius of the external face 8A in the vicinity of the outlet orifices of the channels 71 to 75, is greater than the radius 20 of the upstream part of the duct 7 of annular section, in order to compensate for the

  decrease in free section resulting from the presence of these obstacles.

  With reference to FIGS. 2 and 3 and to FIG. 3 in particular, it can be noted that four channels 111, 112, 113, 114 delimited by partitions are associated with solid fuel passage channel 71 located between two consecutive obstacles 17. 191, 192, 193, 194 which are provided in the annular axial primary air supply duct 11 and allow the formation of four axial air jets associated with the flow of solid fuel leaving the channel 71 to promote the combustion of this fuel ; for this purpose, the first three channels 11 ,, 112, 113 are substantially arranged radially opposite the channel 71 for ejecting solid fuel, and are completed by a fourth channel 114 disposed downstream of the channels 111, 112, 113 by reference in the direction 91 of rotation of the swirling air delivered at the outlet of the burner nozzle by the duct 9 for supplying swirling primary air; in this way, the set of channels 111, 112, 113, 114 associated with channel 71 is angularly offset, in position, with respect to the angular position of this channel, in order to take account of the fuel entrainment effect and of the flame by the swirling air leaving the duct 9, and is therefore offset in the same direction 91 as the direction of the whirlpool. The burner illustrated in FIGS. 2 and 3 further comprises an additional duct 13, also of annular section, coaxially arranged

  to the conduits 3, 5, 7, 9, 11, and to the exterior thereof, which serves to deliver by a peripheral or external annular outlet orifice 130 a flow of oxygen-depleted gas, in particular smoke resulting from combustion.

  This preferred embodiment makes it possible to further reduce the formation of both thermal and combustible NOx for such burners comprising at least two substantially concentric conduits: a fuel supply duct 7 and at least one fuel supply duct 9, 11 primary combustion air representing a maximum of 15% of the total air; these conduits together form a nozzle at the outlet of which the start of combustion can take place and around which secondary air is brought representing at least 85% of the total air and having a temperature of more than 500 ° C .; according to a preferred embodiment, the burner comprises at least four independent supply circuits, one of which supplies the annular duct 13 surrounding all of said ducts 3, 5, 7, 9, 11 for supplying fuel and primary air; inside the duct 13 circulates oxygen-depleted gas, and outside this duct is supplied the secondary air; the inner edge of the outlet port 130 of the conduit 13 is offset radially (that is to say distant from the axis 1) relative to the outer edge of the outlet ports 110 of the concentric conduit 11 (the outermost of the nozzle) by an annular hub 900 of thickness at least

  mm and from 3 to 8% of the diameter of said nozzle.

  According to a preferred embodiment, the exit speed of the oxygen-depleted gas is lower than that (Vc) of the primary air, and of the order of magnitude of that (Va) of the secondary air; in addition, the outlet orifice 130 of the annular duct can diverge and form an angle a, between 2 and 10 relative to the axis 1 of the burner; the oxygen-depleted gas is preferably recycled flue gas, or steam or a mixture of steam and fumes. According to a particular embodiment, said burner may also include, outside the protection 907 (made of concrete) of the outermost concentric duct and towards the outlet orifice thereof, obstacles capable of deflecting and deviating from the axis 1 of the nozzle the flow VA of the secondary air stream 903. The presence of obstacles in the outer ring on the burner on the one hand, and on the other hand the addition of an annular duct 13, which surrounds the the set of fuel and primary air supply conduits, which is supplied with oxygen-depleted gas, and which is disposed at a minimum distance from the outermost primary air supply conduit 11, separate from it ci the secondary air flow15 and makes it possible to delay the exchange of energy, and therefore their mixing, between these various flows: these indeed arrive at speeds

  different, the speed of the secondary air being of the order of 10 to 50 meters per second, while that of the primary air is of the order of 150 to 200 meters per second.

he

Claims (10)

  1. Solid fuel burner which comprises at least one duct (7) of annular section for supplying solid fuel and at least one duct (9, 11) for annular air supply, which is arranged substantially coaxially and outside the duct solid fuel supply, characterized in that it comprises   at least two obstacles (17) provided in the fuel supply duct (7), which divide the duct (7) into at least two channels (71, 72, 73, 74, 75), in the vicinity of the orifice outlet (7).   2. Burner according to claim 1, in which said obstacles (17) are in the form of ribs or thick partitions, of variable cross section, preferably in profile and of continuously variable cross section, for example wedge-shaped and elongated shape. along the common longitudinal axis (1) of the ends of the conduits (7, 9, 11), to limit or avoid abrasion by solid fuel particles, while separating the flow of solid fuel by   several veins clearly separated at the outlet of the nozzle.   3. Burner according to any one of claims 1 or 2,   in which the height of the channels separated by said obstacles (17) from the solid fuel supply duct, is greater than the height of the part of the duct (7) located upstream of the obstacles, to limit the resulting increase in speed of the decrease in   solid fuel free passage section in the channels.   4. Burner according to any one of claims 1 to 3, which   comprises at least three and preferably at most twelve obstacles, in particular from three to six obstacles substantially regularly distributed along the periphery or circumference of the annular duct (7) and arranged near the outlet orifice of the supply duct (7) to allow the formation of at least three flames of similar or identical shape and size and distributed around the longitudinal axis   common (1) according to rotational symmetry.   5. Burner according to any one of claims 1 to 4,   in which the portion of the solid fuel supply duct (7) comprising said obstacles (17) and said channels delimited by   the obstacles, is of substantially frustoconical or flared shape. 6. Burner according to any one of claims 1 to 5, which comprises a duct (11) for supplying axial air ejected by a   outlet (110) in a direction substantially parallel to the common longitudinal axis (1) of the ducts and guided by fins (19) provided in the duct (11), which duct (11) for axial air supply is arranged around the solid fuel supply duct (7) and in which the number of fins (19) is at least equal to the number of obstacles (17) provided in the solid fuel supply duct, to form at at least as many axial air passage channels as the burner comprises solid fuel passage channels, in order to allow the formation of at least one air jet associated with each stream of the outgoing stream of solid fuel. 7. Burner according to claim 6, in which the number of channels and partitions (19) provided in the duct (11) for axial air supply is a multiple of the number of channels (71 ... 75) and obstacles (17) provided in the solid fuel supply duct (7), to allow the formation of several axial air jets associated with each of the   solid fuel streams or streams coming out of the burner nozzle.   8. Burner according to any one of claims 6 or 7,   in which the channels (11 ", 112, 113, 114) delimited in the air supply duct (11) by the partitions (19) are angularly offset with respect to the channels (71) delimited by the obstacles (17) of the solid fuel supply duct (7) in the direction of rotation (91) imparted to the fuel and to the flame by a swirling air flow delivered by an air supply duct (9) provided with fins or deflectors (18), and in which the channels (111, 112, 113, 114) are inclined or divergent with respect to the longitudinal axis (1) of the burner nozzle.   9. Burner according to any one of claims 1 to 8, which   further comprises a peripheral annular conduit (13) for injecting a gas poor in oxygen external to said supply conduits   in primary air. 10. A method of operating an oven equipped with a burner according to any one of claims 1 to 9, in which the   average speed of solid fuel in the duct (7) and in the channels (71 ... 75) separated by the obstacles (17) at a value at least equal to 20 meters per second and at most equal to 30 meters per second, and in which a solid fuel is delivered around the vein   part of the primary air in swirling form and part of the primary air10 in axial form, and maintaining an axial air flow greater than the swirling air flow.